An untagged unconnected node is a node which is not part of any ways or relations (unconnected) and which does not have any tags on it. Since this cannot possibly represent anything, there is no reason for such a node to exist in the data.

If older than several days it is a bug, and should be deleted, although it can be worth trying to understand the reason it was created in the first place.

Where do empty nodes come from

Forgotten tags: Empty nodes might also come from users that created the node and forgot to add tags to it.

License change redaction: In mid 2012 we had a lot of these nodes left over after the ODbL license change redaction bot redacted ways. This nodes represent locations which can be joined up again with a new way, and new tags. As such they may be useful for remapping.

Partial uploads: A line of untagged unconnected nodes may be caused by editors failing to complete an upload of a way pre April 2009. Since the introduction of changesets in April 2009, this is less likely to occur, since the operations to create nodes and then connect them as a way, are rolled into one request and one DB transaction. Explanation: When uploading data to the database there will first be uploaded the nodes and only later the ways where they are contained, hence resulting in empty nodes for the time the upload is not finished. Please consider that big uploads might go on for hours or even days.

Intentional as "fixme" marker: Some users use 3 nodes in a row to express something like "..." (an unsurveyed diverging road or a continuing road not yet in the db).